1. Field of the Invention
The present invention relates to an image forming apparatus, such, as a printer or a copier, and to an image forming method that uses an electrophotographic method or an electrostatic recording method.
2. Description of the Related Art
In recent years, image forming apparatuses that use an electrophotographic method have been made with increased speed and performance, and have also progressed in color image forming technology. Printers and copiers that use a variety of methods to form color images have been brought onto the market. For example, a so-called xe2x80x9cinline-typexe2x80x9d image forming apparatus is known, in which image forming means for a plurality of colors are arranged in a line and toner images that have been formed by each of these image forming means are transferred in order on top of one another onto a sheet (the transfer medium). Image forming apparatuses of the inline type can form color images at high speeds, and so are expected to become the principal type of color printer, in the future. Such inline-type image forming apparatuses are constructed so as to hold and convey a sheet using a belt-shaped conveying means (a sheet conveyor belt) and to transfer toner images of colors on top of one another onto the sheet in order, thereby producing a color image.
In addition, in the inline-type image forming apparatus, the sheet conveyor belt extends in the direction in which image forming stations are arranged and is disposed in contact with each of the image forming stations. To improve the transferring of toner images onto a sheet that is conveyed by the sheet conveyor belt, it is preferable for the sheet to be stably held on the sheet conveyor belt by attraction. For example, by applying an attraction bias (a voltage or current) to the surface of the sheet conveyor belt, a sheet can be stably held on the sheet conveyor belt by attraction.
When forming toner images on both surfaces of a sheet using the inline-type image forming apparatus described above, after the trailing end of a sheet that has had an image formed on a first surface (the front) thereof has passed a fixer that heat-fixes the toner images on the sheet, the conveying direction of the sheet is reversed so that the second surface (the reverse) of the sheet becomes the image forming surface, the sheet is guided to a re-conveying path once again, and the sheet is fed towards the plurality of image forming means for a second time.
When images are formed on both surfaces of a sheet, however, image formation is performed on the second surface of the sheet after the sheet has been heated and subjected to pressure by the fixer, so that there are cases where a peripheral part of the sheet becomes wrinkled. If a sheet in this state is conveyed by the sheet conveyor belt to which an attraction bias has been applied, the wrinkled part of the sheet is strongly held on the sheet conveyor belt by attraction. As a result, as shown in FIGS. 5A and 5B, the wrinkled part)(Sb) of the sheet (S) causes creases to be formed that are directed towards the trailing end of the sheet. Toner images are not transferred to the concave parts of the creases (i.e., the parts where there is no contact between the sheet and the photosensitive drums), so that there are cases where certain areas are left unprinted. It should be noted that concave and convex parts of the creases are depicted in an exaggerated fashion in FIGS. 5A and 5B for the sake of explanation.
Also, when image formation has ended for the first surface (the front), wrinkling occurs in the sheet that has passed the fixer at both sides in the direction that is perpendicular to the conveying direction. The lengths in the conveying direction of the both sides at which such wrinkling occurs will be longer than the length of the central part of the sheet. This means that when image formation is performed on the second surface (the reverse) of the sheet, the wrinkled parts at both sides of the sheet cause creases to be formed concentrated at the trailing end of the sheet. This can result in unprinted areas being left on the sheet at the transfer sections.
The present invention was devised in view of the above problems with the related art, and it is an object of the present invention to provide an improved image forming apparatus and image forming method.
It is a further object of the present invention to provide an image forming apparatus and image forming method that can suppress the formation of creases when forming images on both surfaces of a sheet and can greatly reduce the incidence of unprinted areas.
To attain the above objects, in a first aspect of the present invention, there is provided an image forming apparatus comprising a sheet stacking section in which sheets are stacked each having a first surface and a second surface, a sheet conveying section that conveys the sheets, an image forming section that forms an image on a sheet while the sheet is conveyed by the sheet conveying section, a re-conveying section that again conveys the sheet having an image formed on the first surface thereof to the image forming section, a sheet feeding section that feeds a sheet from the sheet stacking section or the re-conveying section to the sheet conveying section, wherein the sheet is conveyed while it is held between the sheet conveying section and the sheet feeding section, and a controller that controls a feeding speed of the sheet fed by the sheet feeding section, wherein the controller controls the feeding speed of the sheet in a manner such that the sheet is fed at a first speed when the sheet is fed from the sheet stacking section to form an image on the first surface of the sheet, and the sheet is fed at a second speed lower than the first speed and lower than a speed at which the sheet is conveyed by the sheet conveying section when the sheet is fed from the re-conveying section to form an image on the second surface of the sheet.
Preferably, the first speed is approximately equal to the speed at which the sheet is conveyed by the sheet conveying section.
Preferably, the image forming apparatus according to another aspect of the invention further comprises an operation mode setting section that sets an operation mode of the image forming apparatus, wherein the controller is responsive to setting of a first operation mode by the operation mode setting section, for causing the sheet from the re-conveying section to be fed at the first speed, and is responsive to setting of a second operation mode by the operation mode setting section, for causing the sheet from the re-conveying section to be fed at the second speed.
Preferably, the operation mode setting section sets the operation mode based on information relating to the type of sheet.
In a preferred form of the present invention, the image forming apparatus further comprises a detection section that detects a resistance of the sheet, and wherein the operation mode setting section sets the first operation mode when the resistance of the sheet is higher than a predetermined value and sets the second operation mode when the resistance of the sheet is equal to or lower than the predetermined value.
In another preferred form of the present invention, the image forming apparatus further comprises a detection section that detects humidity, and wherein the operation mode setting section sets the first operation mode when the humidity is equal to or lower than a predetermined value and sets the second operation mode when the humidity is higher than the predetermined value.
Preferably, the image forming section comprises an image carrier that transfers a toner image onto the sheet, and a fixing section that fixes the toner image onto the sheet.
In a specific form of the present invention, the fixing section comprises a pair of rollers that rotate while they are in contact with one another and fix the toner image onto the sheet by heating and applying pressure to the sheet.
Preferably, the sheet conveying section comprises a belt-shaped conveying member that conveys the sheet, and the image forming apparatus comprises a bias applying section that applies a bias to the belt-shaped conveying member to hold the sheet on the belt-shaped conveying member by attraction.
Also preferably, the sheet conveying section comprises a belt-shaped conveying member that conveys the sheet, and the image forming section comprises a plurality of image carriers that are arranged in a conveying direction of the sheet, for forming toner images of different colors on the sheet that is conveyed by the belt-shaped conveying member.
More preferably, the image forming apparatus comprises a bias applying section that applies a bias to the belt-shaped conveying member to hold the sheet on the belt-shaped conveying member by attraction.
To attain the above objects, in a second aspect of the present invention, there is further provided an image forming method of forming an image on both surfaces of a sheet, comprising a supplying step of supplying a sheet from a sheet stacking section in which sheets are stacked each having a first surface and a second surface, a first feeding step of feeding the sheet supplied in the supplying step at a first speed, a first conveying step of conveying the sheet fed in the first feeding step, starting from before feeding of the sheet in the first feeding step is completed, a first image forming step of forming an image on the first surface of the sheet while the sheet is conveyed by the first conveying step, a re-conveying step of re-conveying the sheet having the image formed on the first surface in the first image forming step, in order for an image to be formed on the second surface of the sheet, a second feeding step of feeding the sheet re-conveyed in the re-conveying step, at a second speed that is slower than the first speed and is slower than a speed at which the sheet is conveyed in the first conveying step, a second conveying step of conveying the sheet fed in the second feeding step, starting from before feeding of the sheet in the second feeding step is completed, and a second image forming step of forming an image on the second surface of the sheet while the sheet is conveyed in the second conveying step.
Preferably, the first speed is approximately equal to a speed at which the sheet is conveyed in the first conveying step and the second conveying step.
Also preferably, the first image forming step and the second image forming step each comprise a transfer step of transferring a toner image onto the sheet, and a fixing step of fixing the toner image on the sheet.
More preferably, the first image forming step and the second image forming step each include a plurality of the transfer steps of transferring toner images of different colors onto the sheet.
Preferably, the first image forming step includes a first bias applying step of applying applying a bias for holding the sheet on the belt-shaped conveying member by attraction in the first conveying step, and the second image forming step includes a second bias applying step of applying a bias for holding the sheet on the belt-shaped conveying member by attraction in the second conveying step.
To attain the above objects, in a third aspect of the present invention, there is provided an image forming apparatus comprising a sheet stacking section in which sheets are stacked, a section that has a belt-shaped sheet conveying member that conveys the sheets, an image forming section that forms an image on a sheet while the sheet is conveyed by the sheet conveying section, a re-conveying section that again conveys the sheet having an image formed on the first surface thereof by the image forming section to the image forming section, wherein the sheet is conveyed while it is the sheet conveying section and the sheet feeding section, an attraction section that holds the sheet on the conveying member by a predetermined attraction force, a controller that controls the image forming apparatus, and wherein the controller provides control such that the predetermined attraction force is set to a smaller value when the sheet is fed from the sheet stacking section to form an image on the second surface of the sheet than a value when the sheet is fed from the re-conveying section to form an image on the second surface of the sheet.
Preferably, the attraction section applies a bias to the sheet to hold the sheet on the conveying member by attraction, and wherein the controller provides control such that the bias is set to a smaller value when the sheet is fed from the sheet stacking section to form an image on the second surface of the sheet than a value when the sheet is fed from the re-conveying section to form an image on the second surface of the sheet.
The above and other objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.